Recovery of zinc by electrolysis



R. H. STEVENS.

RECOVERY OF ZINC BY ELECTROLYSIS. APPLICATION FILED NOV. 11. 1919.

1 ,347, 18 9 Patented July 20, 1920.

INVE/VT'OR Roy/2L5 HILL-MAN STEVE/v.5

UNITED STATES PATENT OFFICE.

ROYALE HILLMAN STEVENS, 0F RISDON ROAD, HOBART, TASMANIA, AUSTRALIA,ASSIGNOR T0 ELECTROLYTIC ZINC COMPANY OF AUSTRALASIA PROPRIETARYLIMITED, OF MELBOURNE, VICTORIA, AUSTRALIA.

RECOVERY OF ZINC BY ELECTROLYSIS.

Specification of Letters Patent.

Patented July 20, 1920.

Application filed November 11, 1919. Serial No. 337,320.

To all whom it may concern:

Be it known that I, ROYALE HILLMAN S'rnvENs, a citizen of the UnitedStates of America, residin at care of Electrolytic Zinc Company ofAustralasia Proprietary Limited, of ltisdon Road, Hobart, in the Stateof Tasmania, Commonwealth of Australia, have invented certain new anduseful Improvements in the Recovery of Zinc by Electrolysis, of whichthe following is a specification.

This invention relates to certain improvements in the recovery of zincby electrolysis and refers more especially to the electrodeposition ofzinc from zinc sulfate solutions obtained by leaching ores, concentratesor other zinc bearing materials with sulfuric acid (or spentelectrolyte.)

In the recovery of zinc by electrolysis the zinc is deposited oncathodes preferably of aluminium. The deposition is allowed to continuefor certain periods,- when the metallic zinc is stripped at requiredintervals.

These cathodes usually consist of sheets of aluminium which aresuspended in the electrolytic cells from copper supporting bars to whichthe aluminium sheets are riveted, the said copper bars formingelectrical contact with the busbars through which the electric currentis led to the cells.

These copper protecting bars are liable to corrosion from the acidsprays arising from the cells causing also contamination of the cellsolution and are therefore as a rule protected by being covered withsheet lead where they overhang the cell.

The cathode sheets are immersed in the solution within the cells leavinga space of exposed aluminium between the surface of the solution and thesupporting bars which surface is found to be subject to corrosion bothon account of the creeping upward of the acid solution by capillarityand by the action of the acid sprays arising from the cells duringelectrolysis sov that the alu minium is found in practice to corrodemost rapidly at this part.

The object of this invention is to provide means whereby these exposedsurfaces of" the cathodes may be protected from corrosion and therebyprolong their life and usefulness in the electrodeposition of zinc.

I accomplish this object by providing arranged in such a manner that theordinary deposit obtained from the cell itself is easily stripped alongthe line of the said solution level from time to time as required.

For the purpose of more clearly illustrating the application of thisinvention reference may be made to the accompanying sheet of drawingswhich shows diagrammatically a cathode A suspended in a cell B andsupported from bars A. The cathode A is preferably of,sheet aluminiumand the supporting bars A are usually of copper adapted to rest upon thebusbars B. These supporting bars A are usually sheathed in lead, (asshown) for the purpose of protecting the copper from acid spray andavoiding contamination of the solution with copper salts. The line a-brepresents the normal solution level of the cell. In the adaptation ofthis invention to the practice of the electrodeposition of zinc aprotective coating of zinc is applied to the cathode sheets from aboutthe line a--b to just below the edge of the lead sheathing around thesupporting bars A-say on the line ef.

In carrying this invention into effect the cathode sheets are firstthoroughly cleansed, and scrubbed with sandstone or wire brushes so asto slightly roughen the surface of the upper part of the cathode wherethe protecting deposit is required to adhere firmly. Two Wooden stripsabout three quarters of an inch wide are then placed across the face ofthe sheet and held in place by rubber bands or clips the upper edge ofthe said wooden strips coinciding approximately with the normal solutionlevel of the cell (line a-b). The cathode plates with their woodenstrips as indicated are then placed in a special cell in which thepurified solution is at a higher level than in the ordinary cells beingjust below the lead covering of the supporting bars A.

The deposition of zinc is then proceeded with for the requisite period,say 24 hours at 25 to 30 amperes per square foot current density, oruntil a sufficient deposit has been formed to provide the requiredprotective coating. The cathodes are then lifted from the cell, cleaneddown and the wooden strips removed, and after drying, any exposed partsof the aluminium (particularly just below the lead covering of thesupporting bars) are covered with a protective covering of pitch and tar(applied hot) varnish or other material of a like nature.

The wooden strips prevent the deposition of the protecting coating ofzinc over the area thus covered so that after the cathode plate has beenprotectively covered and subsequently used for deposition of zinc in theordinary way there will be a line of weakness along the line ab wherethe subsequent deposit of zinc meets this protecting deposit.Consequently when the stripping is elfected later the zinc deposit willpart, readily along this line a-b and peel away from the cathode withoutremoving or disturbing the protecting deposit of zinc in the upperexposed part of the cathode above the line w-b.

The cathodes A with their protective coating are then used for theelectrodeposition ofthe zinc in the ordinary way.

In practice it is sometimes found that the protective coating breaksaway slightly when stripping, at or about the line ab and for thepurpose of restablishing the coating it is sometimes desirable to aflordmeans for the deposition of zinc above the line of the -normal solutionlevel -(ab) and for this purpose a temporary bafile or the like may beinserted in the effluent lip of the cell with a view to. raisingthe'solution level-for the time being to about the line cd. Theelectrodeposition is proceeded with for a few hours at this higher leveluntil the protective coating is reestablished when the level is againdropped to the normal line a-b by removing the baffle or'obstructionfrom the efliuent lip of the cell and the electrodeposition proceededwith in the ordinary way.

The protecting of the cathode plate in this way affords a deposit uponthe surfacecomplete protection to the exposed parts and furthermore doesnot introduce into the solution any foreign material which wouldinterfere with the electrolytic process.

I claim 1. In the electrolytic process for the recovery of zincproviding a protective coating of zinc on the surface of the cathodesabove the solution level whereby the cor-.

roding action of the acid sprays is substantially eliminated.

2. In the electrolytic process for the recovery of zinc providing aprotective coating of zinc on the surface of the cathodes by firstelectrolytically depositing a thin strips affixed at or about the normalsolution level which prevent the deposition of zinc under them, and thenremoving the said. strips and proceeding with the electrolysis in theordinary Way in cells with the solution at normal level whereby thesubsequently deposited zinc strips along the lineof weakness at or aboutthe solution level; substantially as described.

4. In the electrolytic process for the recovery of zinc providing aprotective coating above the normal solution level substantially asclaimed in claim 3 and reestablishing the said coating at or about thesolution level by temporarily raising the solution level and proceedingwith electrolysis and then loweringthe level to the normal line andproceeding with electrolysis in the ordinary way; substantially asdescribed.

In testimony whereof I have signed my name to this specification in thePresence of two subscribing witnesses.

ROYALE HILLMAN STEVENS.

Witnesses:

.ALLAN- MCINTY E, V. J. CHAMBnRs.

